We are continuing our analysis of electrophoretic variants of enzymes and their inheritance in various populations of frogs (genus Rana) and fish (genus Xiphophorus). In embryos heterozygous at polymorphic enzyme loci the time of gene expression is detected by appearance of maternal and hybrid enzyme forms. The effects of nuclear- cytoplasmic interaction on the qualitative and quantitative expression of enzyme phenotypes are investigated in diploid and haploid hybrid frog embryos. Control mechanisms operating on the expression of specific enzyme loci are being studied by microinjection of drugs affecting macromolecular synthesis and subcellular fractions from eggs and embryos.